SLUDGE DEWATERING: THE CAPE TOWN PERSPECTIVE APPROPRIATE TECHNOLOGY CONFERENCE Presented by: KEVIN FAWCETT Date: Tuesday, 4th September 2007

INTRODUCTION Past experience was that sludge disposal would look after itself The reality that sludge handling could possibly account for as much as 50% of operating costs was ignored Many WWTW in Cape Town relied entirely on sludge lagoons and sludge drying beds as the primary option for sludge dewatering In 1995 the then Cape Town City Council approved the installation of plant to effect sludge thickening, dewatering and thermal drying of the sludge at Cape Flats WWTW, some 20 years after the WWTW was commissioned

Cape Flats WWTW: 200 Ml/day

Original drying beds

CFWWTW: Sacrificial land disposal

CFWWTW: Sludge Management Options

CFWWTW: Sludge solids mass balance

Estimate of operating costs for each option: 1995 Costs Process Description Rand/ton # 1 Disposal(Vissershok) 2229 2 Dewatering/composting/disposal(Vissershok) 854 3 Dewatering/drying(60%DS)/composting/disposal(Swartklip) 741 4 Dewatering/drying(60%DS)/disposal(Swartklip) 683 5 Dewatering/drying(90%DS)/disposal(Swartklip) 622 6 Digestion/dewatering/composting/disposal(Swartklip) 534 7 Pre-thickening/digestion/dewatering/composting/disposal(Swartklip) 506 8 Dewatering/disposal(Vissershok) 448 9 Dewatering/private disposal 382 10 Digestion/dewatering/drying(90%DS)/disposal(Swartklip) 327 11 Digestion/dewatering/drying(60%)/composting/disposal(Swartklip) 315 12 Pre-thickening/digestion/dewatering/drying(60%DS)/composting/disposal(Swartklip) 294 13 Pre-thickening/digestion/dewatering/drying(90%DS)/disposal(Swartklip) 295* 14 Digestion/dewatering/disposal(Swartklip) 273 15 Digestion/dewatering/drying(60%DS)/disposal(Swartklip) 269 16 Pre-thickening/digestion/dewatering/disposal(Swartklip) 260 17 Pre-thickening/digestion/dewatering/drying(60%DS)/disposal(Swartklip) 250

CENTRIFUGES SLUDGE THICKENING & DEWATERING

DEWATERED SLUDGE INLET PELLETS OUTLET DRYING DRUM DEWATERED SLUDGE INLET THERMAL DRYING PLANT

Process Considerations when selecting centrifuges Anaerobic digestion of waste activated sludge prior to dewatering could present problems with respect to Struvite precipitation Screening of sludge is necessary to prevent accumulation of fibre in the centrifuge bowl Installation of tungsten carbide tiles should be a necessity if the feed sludge is in any way abrasive

Struvite

ANAEROBIC DIGESTED SLUDGE 4 to 5% Dry Solids

STRAINPRESSES - REMOVAL OF FIBRE AND FOREIGN OBLECTS FROM SLUDGE

Strainer Boxes Lifting chain 6mm perforations

Centrifuges can be problematic Poor operation, grit and fibre in sludge Not following the suppliers operating and maintenance instructions

Tungsten Carbide Wear Tiles

Centrate & sludge cake inspection Points Regular wear check profiles of the scroll are essential

Macassar WWTW

Wildevoelvlei WWTW

Incorrect belt tensioning

Foreign objects between belts

Roller failure

Roller failure due to poor manufacture

In a number of instances old established technologies are still appropriate Biofilter Plant

However the same does not apply to activated sludge plants- high sludge production rates Winter sludge disposal costs = R0,75m

Conveyance of dewatered sludge cake: What is the appropriate technology? Screw Conveyors: Centreless or Shafted Progressive Cavity Pumps The following slides indicate some short comings and problems and where conveyors are appropriate

Where sludge storage silos are not required, conveyors and skips are appropriate

Accessibility for maintenance Failure of silo screw conveyors= empty the silo

Old versus upgrade version

Sludge cake transfer to disposal skip

Where do we go from here?

Athlone WWTW: Anaerobic digesters/ drying beds

Possible Solution? Progressive Cavity Pump

Dewatered cake and silo pumps Can insert a knife gate valve for installation at the base of sludge storage silo Advantage = In the event of repairs to pump can isolate silo without having to empty it

Level sensors Pumped discharge

Proposal for ATHLONE WWTW Progressive cavity pumps Centrifuges

Conclusion Have we made the correct decision – time will tell What ever dewatering technology is selected it is essential that the appropriate operating and maintenance skills are in place before the plant is commissioned Consider the possibility of entering into a maintenance agreement with the equipment supplier to ensure sustainability of the equipment All Metro’s and Municipalities have between them a multitude of varied experiences in the wastewater industry, both good and bad – it is time that we engaged in open discussions with each other to learn from each others experiences and hopefully the selection of the “appropriate technology” will become easier.